Propeller blade construction



Get. 12, 1948. E. MARTIN PROPELLER BLADE CONSTRUCTION Filed April 22, 1944 M 7 Rm w W G w P R c T E H T C .a VM T E M/ D 7 m E J/ NM 2 In ,7 A F 3 F M v 7 Y. V B a w Q\WMHW @\Q2U h v /W in MN c a 4 M M cw X D N Um m .\V/ F w W C Patented Oct. 12, 1948 UNITED STATES PAT 2,451,106 it i a j I PROPELLER BLADE CONSTRUCTION I Erle Martin, West Hartford, Conn. assignor to United Aircraft Corporation, East Conn.', a corporation of Delaware 7 Application April 22, 1944, Serial bid 532,246

3 Claims. (c1. 170;:159

This application is a continuation-in-part of U. S. application Serial No. 416,058, filed October 22, 1941 (now abandoned) by Erle Martin for Propeller blade constructions.

This invention relates to improvements in aeronautical propellers and has particular reference to an improved form of metal blade for such a propeller.

An object of the invention resides in the provision of an improved aeronautical propeller having metal blades which will not be materially weakened by the scratches and abrasions to which the faces of such blades are subject in operation.

A further object resides in the provision of an improved aeronautical propeller having metal blades which are initially curved to relieve destructive stresses in certain portions of the blade during propeller operation.

Other objects and advantages will be more particularly pointed out hereinafter or will become apparent as the description proceeds.

In the accompanying drawing in which like reference numerals are utilized to designate similar parts throughout, there is shown a suitable mechanical arrangement for the purpose of disclosing the invention. The drawing, however, is for the purpose of illustration only and is not to be taken as limiting the invention since it will be apparent to those skilled in the art that various changes in the illustrated construction may be resorted to without in any way exceeding the scope of the invention.

In the drawing,

Fig. 1 is a diagrammatic side elevation of a propeller, the initial deformity of the blades being somewhat exaggerated for the sake of clearness in the illustration.

Fig. 2 is a diagrammatic front elevation of the propeller of Fig. 1.

Fig. 3 is a transverse sectional view on the line 3-3 of Fig. 1 looking in the direction of the arrows.

Fig. 4 is a Goodman diagram illustrating the difference in vibratory stress carrying ability of surfaces under different amounts of tension and compression.

The two sides of a propeller blade are called the thrust face and the camber face or side. The thrust face, which is located on the rearward side of the blade relative to the direction of travel of the aircraft on which the propeller is mounted, is the face which comes in contact with solid particles in the air. These particles such as pieces of stone or cinders picked up from the runways cause scratches or nicks in the thrust face. This abrasion in the form of scratches or nicks greatly reduces the fatigue strength of the propeller blade particularly when the nicked portion is in tension. I have discovered, however, that if the abraded surface is maintained under indicate tension in the blade.

7 Hartford;

compression, the fatigue strength of the abra d surface is not reduced to the same extent as where'the surface is maintained under tension. fA blade designed so that the surface subject to, abrasion is always under compressionduring o eration, could, therefore, bedepended uponlto' carry higher stresses or a smaller amount of blade" material could be used'than in a bladeide si ned to have the abraded surface under tension'lff" In order to accomplish the object of maintai'ne, ing the thrust face under a steady compressiom the propeller blades arecurvedI-away from and substantially normal to the. thrustffacef25,'. as' shown in Figs. 1 and 2. This gives a blade which curves forward with respect to the direotionfof travel of the aircraft on which thepropellerlis mounted and backward with'respect to'thedirec' tion Ofro-tation of the propeller. The metal propeller blades, which are indicated by the numr; als 22 and 24, are secured to a hubl'6 mounted: on a. drive shaft I8, projecting from a propeller driving engine 20. Centrifugal forceiwill tendto straighten blades curved as shown infFigs. 1' and 2', thus placingthe' camber surfaces .23 under increased tension and the, thrustfaces ,25 under compression.

The relation between the fatiguei strength "of members under tension and members underl'compression is clearly illustrated in the Goodman diagram, Fig. 5. In this diagram .the' vertical lines to the right of line l0 andfnumbered l to 1 indicate the steady tension load in a bladeJ The horizontal lines above the line I2 and extending to the right of line l0 and numbered l to I also This diagram is read by moving to the right along the line |2 to a point which represents the steady tension in the blade, then traveling upwards from that point 'to the intersection with lines C, C of the diagram; The points represented by these intersections are the safe limits of the vibratory stress of an abraded surface under the selected steady load. Thus, selecting 10,000 lbs. per sq. in., represented by numeral I, as the steady load, it is found that this line intersects the lines C, C at about 1,000 and 19,000 lbs. per sq. in. sothat the safevibrag tory stress, as indicated by the line A, would be between 1,000 and 19,000 lbs. per sq. in. ore total range of 18,000 lbs'. per sq. in. Anotherway of explaining the relation would be .to consider that the steadyload of 10,000 lbs. per sq. in. has superimposed upon it a varying load 'of plus 9,000 lbs. per sq. in. and minus 9,000 lbs'.;per sq. in. giving final resultant loads of 1,000 lbs.

per sq. in. and 19,000 lbs. per sq. in. as the safe mine the locus of the centers of gravity of the various blade sections. In solving this formula S is given a value which is the steady compressive stress selected as the desirable quantity to be maintained in the thrust face at operating conditions.

This formula is based on conditions existing when the blade is at substantially O pitch and the curvature is, therefore, substantially all in a direction parallel to the axis of rotation of the propeller and there is substantially no curvature in the direction of the plane of rotation of the propeller. While the conditions which exist when the propeller pitch is changed to a value used in normal operations will differ from the conditions which exist at O pitch, the change in forces acton the blade and tending to produce tension or compression in the faces thereof are such that their effect is substantially balanced and the resultant effect on the blade is substantially that determined by the above-identified formula. Hence, blade curved to an extent at least as great as the line determined by solution of the above formula will have its thrust face maintained under the selected desired stress during selected operating conditions.

It is to be understood that the invention is not limited to the specific embodiment herein illustrated and described, but may be used in other ways without departure from its spirit as defined by the following claims.

I claim:

1. An aeronautical propeller having a hub and metal blades extending substantially radially outward from said hub, said blades having a thrust face and being initially curved away from said face to an extent at least as great as the extent etermined by the formula I=minor moment of inertia of section at radius X A=area of section at radius X C=distance from neutral axis to outermost fibre on face side at radius X t=tota1 blade air load per in. of radius at maximum operatin power and thrust conditions for radius X Fc=centrifugal stress at station =i g mdw S=desired face side compressive stress at station X M desired bending moment:

%s), at station X E=modulus of elasticity Yo=desired offset at any station X w=rotative speed radians per/sec. x=radius of section being considered R=radius of blade tip y=mass density of blade material 2. A metal propeller blade having a longitudinally extending concave forward surface and a longitudinally extending convex rear face surface and having the centers of gravity of its various transverse sections located on a line which curves in a direction away from said rear face surface as it extends from the shank to the tip portion of the blade, said blade adapted to be mounted in a blade supporting hub so as to extend substantially radially from said hub adjacent the hub with the outer portion of the blade curved forward to an extent that, as a result of the action of centrifugal force on the blade, tending to straighten the blade and force it into a radial position during operation of the propeller in the operating speed range, the predominant stress acting on the rear face surface of the blade is a compressive stress, said forward curvature being limited so that the tensile stress in the front face of the blade is within the allowable working stress of the blade material when the propeller is operated within said speed range.

3. An aeronautical propeller having an operating speed range and having a hub and metal blades extending substantially radially outward from said hub, each blade having a thrust face and. being initially curved away from said face to render the same convex, thus providing a blade curved axially forwardly and curved rearwardly with respect to the direction of rotation, said curvature being greater than the amount necessary to neutralize, by the effects of centrifugal force, the thrust induced bending forces acting on said blade when operating in said speed range, said curvature being such that said blades are substantially straightened by centrifugal force acting on said blade during operation within said range, said straightenin producing a compressive stress in said thrust face of said blade greater than the tension stresses induced in said face, during operation within said range, by thrust forces and centrifugal forces, and said curvature being less than the amount at which the tensile stress in the opposite face of the blade, when operating within speed range, would exceed the allowable working stress of the blade material.

ERLE MARTIN.

REFERENCES CITED The following references are of record in the file of this patent: l

UNITED STATES PATENTS 

